[Volume Control] Only initialize view once

Only initialize viewController once, the first time the volume control
is opened. Re-initializing it can cause an error assigning it as first
responder to the volume slider popover view.

Signed-off-by: Christopher Snowhill <kode54@gmail.com>

Window/VolumeButton.m

@@ -38,9 +38,11 @@
 - (void)mouseDown:(NSEvent *)theEvent {
 	[popover close];
 
+	if(!viewController) {
 		viewController = [[NSViewController alloc] init];
 		viewController.view = _popView;
 		popover.contentViewController = viewController;
+	}
 
 	[popover showRelativeToRect:self.bounds ofView:self preferredEdge:NSRectEdgeMaxY];